Acid gas removal

ABSTRACT

The use of turbulent contactors to simultaneously absorb selected acid gas components from a gas stream and flash off hydrocarbons entrained in a liquid stream which include a solvent or reagent for the selected gas component. In particular, the method comprises feeding the gas stream and liquid stream into a first contactor where they are contacted co-currently and subjected to turbulent mixing conditions, and passing the multi-phase flow from the first contactor to a second contactor. The multi-phase flow from the second contactor is subsequently separated into a gas phase and a liquid phase.

RELATED APPLICATION

This application claims priority to PCT Application No. PCT/GB02/05878filed Dec. 20, 2002 and Great Britain Application No. 0130792.5 filedDec. 21, 2001.

TECHNICAL FIELD

The present invention relates to a method for the simultaneous removalof acid gas components and hydrocarbons from a gas stream. In particularthe invention relates to the selective removal of H₂S over CO₂ using anamine.

BACKGROUND ART

Conventionally, acid gas components are removed using large absorptioncolumns. These columns are designed to handle a gas feed at a certainflow rate and acid gas content. However changes to the feed conditionscause problems in operation and the absorber may have to besubstantially modified to attain satisfactory or optimum performance.Problems where modification is required include, but are not limited to:an increase in the volumetric sour gas flow rate; a requirement for alower acid gas concentration in the treated gas; a lower solventcirculation rate for the same level of purification; an increase in theacid gas concentration in the feed gas; any combination of the above.These problems with existing plants no longer being able to cope withchanged working requirements may be summarised as bottlenecking andtheir solution as de-bottlenecking.

Modifications to the existing absorption columns and the associatedapparatus are expensive and time consuming and many of the abovevariables can change on a regular basis. One of the current solutionsemployed to address the problem of needing to increase acid gas treatingcapacity is to change the solvent employed. However, this is not alwaysappropriate and can introduce secondary problems such as regenerationconsiderations and corrosion problems. Another solution is to change thecolumn internals from, for example, plates to random or structuredpacking. This option only has limited capacity for increasing the acidgas removal due to the overall size of the column.

The present invention therefore provides a method for increasing theacid gas absorption capacity of an existing plant and therebyde-bottlenecking plants.

A further problem with plants which have become bottlenecked is thathydrocarbons and carbon dioxide may become entrained and/or absorbed inthe acid gas solvent (e.g. amine) and are therefore subsequently fed tothe downstream treatment units such as a Claus sulphur recovery unit.These additional components in the feed to the Claus unit reduce theefficiency of the de-sulphurisation plant and may cause an additionalbottleneck further down the process. There are also additional loads onthe system involved in pumping this extra gas round the acid-gas solventregeneration system and de-sulphurisation plant. This may overload theexisting pumps and require the addition of more or replacement pumps.

SUMMARY OF THE INVENTION

The present invention provides a method for increasing the acid gasabsorption capacity of an existing plant and thereby de-bottleneckingplants. In addition, the present invention provides a process in whichthe co-absorption of carbon dioxide and/or the entrainment ofhydrocarbons is reduced or eliminated.

There is therefore a need for a process in which the co-absorption ofcarbon dioxide and/or the entrainment of hydrocarbons is reduced oreliminated. The present invention provides a method for doing this.

According to the present invention, there is provided a method forsimultaneously absorbing selected acid gas components from a gas streamand flashing off hydrocarbons entrained in a liquid stream including asolvent or reagent for the selected gas component, in which: the gasstream and liquid stream are fed into a first contactor where they arecontacted co-currently and subjected to turbulent mixing conditions; themulti-phase flow from the first contactor is passed to a secondcontactor comprising a housing adapted to be inserted into a pipe and tohave a fluid flow pass therethrough, said housing comprising an inletand an outlet opening respectively, the housing being provided with atleast one interior moveable sealingly mounted regulating elementpartially enclosing a central chamber, to provide first wall portionsassociated with an upstream side of said housing, and second wallportions associated with a downstream side of said housing, said wallportions being provided with a number of through-going flow channels,each having a substantially smaller cross-sectional area than the flowcross-section of the inlet and outlet openings respectively, and inwhich the regulating element is adapted to be moveable in relation tosaid housing; and the multi-phase flow from the second contactor isseparated into a gas phase and a liquid phase after the secondcontactor.

Using the method of the present invention, significant reductions in thequantity of hydrocarbon entrainment in the liquid stream can be obtainedwithout any loss in performance of acid gas absorption. It is thereforepossible to use semi-loaded liquid streams which have hydrocarbonsentrained for the treatment of gas streams for the absorption of acidgas components. The method also provides benefits in the removal of thehydrocarbons thereby minimising problems further downstream wherehydrocarbons may block the system or cause a further bottleneck.

Optionally the first contactor is a turbulent contactor having acontracting pipe section through which a gas stream flows, a liquidinlet arranged to produce an annulus of liquid around the internalperimeter of the pipe, a sharp edge at the end of the contracting pipeand a further pipe section downstream of the sharp edge.

Alternatively, the first contactor may comprise a vessel including a gasinlet, a liquid inlet and an outlet, the outlet leading to a venturipassage and a tube extending from the outlet back upstream, the tubebeing perforated and/or being spaced from the periphery of the outlet.

Alternatively, the first contactor is the same as the second contactor.

Preferably, H₂S is selectively absorbed in preference to CO₂ from thegas stream. More preferably, the H₂S level in the output gas stream fromthe second contactor is less than 1.5% by volume, even more preferablyless than 1% by volume.

Preferably, the liquid stream is an amine stream including entrainedhydrocarbons. Optionally, the liquid stream (including entrainedhydrocarbons) is fed directly to the first contactor from a liquefiedpetroleum gas (LPG) de-sulphurisation train. Preferably, the amine isselected from MEA, DEA, DIPA and MDEA.

Preferably, the liquid phase is treated to remove any absorbed gascomponent. This treatment may be by any suitable means as are known. Thetreated amine, now clear of sour gas components, may be recycled to thegas treatment system. The sour gas mixture from the treated amine may bepassed to any suitable downstream treatment plant such as a Claus plantto convert the H₂S into environmentally acceptable products.

Preferably, at least 70%, more preferably 80% and more preferably still90% of the hydrocarbons in the liquid feed are flashed off into the gasstream in the contactor unit.

The present invention also extends to the apparatus for use in themethod of the present invention. In particular, according to the presentinvention there is provided apparatus for the simultaneous absorption ofselected acid gas components from a gas stream and flashing offhydrocarbons entrained in a liquid stream including a solvent or reagentfor the selected gas component comprising: a first co-current contactorwhere the gas stream and the liquid stream are subjected to turbulentmixing conditions; a second co-current contactor comprising a housingadapted to be inserted into a pipe and to have a fluid flow passtherethrough, said housing comprising an inlet and an outlet openingrespectively, the housing being provided with at least one interiormoveable sealingly mounted regulating element partially enclosing acentral chamber, to provide first wall portions associated with anupstream side of said housing, and second wall portions associated witha downstream side of said housing, said wall portions being providedwith a number of through-going flow channels, each having asubstantially smaller cross-sectional area than the flow cross-sectionof the inlet and outlet openings respectively, and in which theregulating element is adapted to be moveable in relation to saidhousing; and means for separating the multi-phase flow from the secondcontactor into a gas phase and a liquid phase.

Optionally, the first contactor is a turbulent contactor having acontracting pipe section through which a gas stream flows, a liquidinlet arranged to produce an annulus of liquid around the internalperimeter of the pipe, a sharp edge at the end of the contracting pipeand a further pipe section downstream of the sharp edge.

Alternatively, the first contactor comprises a vessel including a gasinlet, a liquid inlet and an outlet, the outlet leading to a venturipassage and a tube extending from the outlet back upstream, the tubebeing perforated and/or being spaced from the periphery of the outlet orthe first contactor is the same as the second contactor.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention may be put into practice in various ways and a number ofspecific embodiments will be described by way of example to illustratethe invention with reference to the accompanying drawings, in which:

FIG. 1 shows a process configuration for a portion of a possibleexisting acid gas treatment plant which is bottlenecked;

FIG. 2 shows a process configuration exemplifying the present inventionutilising solvent integration;

FIG. 3 a shows a mixer used as the second mixer and optionally also usedas the first mixer in the method of the present invention, the viewbeing an axial longitudinal section normal to a common axis of rotationin the mixer;

FIG. 3 b shows the mixer of FIG. 3 a in axial longitudinal section butcoincident with the axis of rotation;

FIG. 3 c shows a cross section of the mixer of FIG. 3 a through thecommon axis of rotation;

FIG. 4 a is a view of a turbulent contactor suitable for use as a firstmixer in the method of the present invention;

FIG. 4 b is a schematic representation of the break up of the liquidfilm into droplets in the 10 turbulent contactor of FIG. 4 a;

FIG. 4 c is a view of a second embodiment of a turbulent contactorsimilar to that of FIG. 4 a suitable for use in the method of thepresent invention;

FIG. 4 d is an enlarged view of the liquid stream inlet portion of thecontactor shown in FIG. 4 c ringed in circle A;

FIG. 5 a is a view of another contactor suitable for use as a firstmixer in the method of the present invention;

FIG. 5 b is a variant of the contactor shown in FIG. 5 a;

FIG. 5 c is a view of a contactor similar to that shown in FIG. 5 a butwith the perforated tube arranged so that all the fluid which passesthrough the outlet does so by way of the tube;

FIG. 5 d is a variant of the contactor shown in FIG. 5 c;

FIG. 6 shows the set-up used to measure the simultaneous performance ofthe contactor unit on removal of LPG from an amine feed and selectiveH₂S absorption from an off-gas feed;

FIG. 7 shows the results of a calibration test of the LPG measurementapparatus;

FIG. 8 shows the HC content in the amine stream before and afterprocessing in the contactor unit; and

FIG. 9 shows the H₂S and CO₂ content of the off-gas feed against timeafter it has been fed to the contactor unit.

DETAILED DESCRIPTION

FIG. 1 shows an example of an acid gas treatment plant which isbottlenecked. A conventional counter-current absorption column 1 is fedwith lean amine solvent via stream 3 and with a feed gas via stream 5.The treated gas is removed via stream 7 and may be subject to furthertreatment as is known. The used amine is removed via stream 9 and is fedto an amine regeneration unit 11.

The lean amine solvent is also fed to a liquefied petroleum gas (LPG)treatment unit indicated by the box 20 in FIG. 1. The amine is fed viastream 21 to a counter-current column 22 to which LPG gas is also fedvia stream 23. The treated gas is fed to a mixer 24 via stream 25 whereit is mixed with further lean amine fed via stream 26 before beingpassed to a settler 27 where the two-phase mixture is allowed to settle.Some of the removed used amine may be fed back to the column 22 viastream 28, but most of it is passed to the amine regeneration unit 11via stream 29. Used amine from the column 22 also passes directly to theregeneration unit 11 via stream 30. The gas stream from the settler isthen passed to a coalescer 31 via stream 32. The treated gas is thentaken off via stream 33 and may be subject to further treatment or useddirectly as appropriate. The liquid stream 34 from the coalescer 31 ispartly recycled to the coalescer via stream 35 and partly sent to theregeneration unit 11 via stream 36. It can be seen that a significantquantity of amine is used once and subsequently sent directly to theregeneration unit. This produces a heavy liquid load on the system whichis controlled by pumps and valves (not shown).

FIG. 2 shows a similar operating system in which a unit according to thepresent invention is added before the conventional counter-currentcolumn 1 for the treatment of the feed gas. Instead of streams 29, 30and 36 passing directly to the amine regeneration unit 11, they are fedto an additional contactor unit 50 via stream 51. Some will still bepassed directly to the regeneration unit 11 via stream 52. The contactorunit 50 is also fed by some of the used amine from the contactor 1 viastream 53. This semi-lean amine from the four sources is fed to thecontactor unit 50 where it is contacted co-currently with the feed gas5. The unit comprises two mixers, the second of which is an in-linemixer such as that shown in FIGS. 3 a, 3 b and 3 c. The first mixer maybe any suitable contactor and the selection may be dictated by variablessuch as available space, liquid flowrate, gas flowrate etc.

After the second mixer the two-phase mixture is coarsely separated, theloaded amine being passed to the regeneration unit 11 via stream 54 andthe treated gas being passed to the existing column 1 via stream 5 a.The separation in contactor unit 50 is such that less than 1% of theliquid is entrained with the separated gas and negligible gas isentrained with the separated liquid.

Treatment in the contactor unit 50 increases the loading on the aminebefore it is passed to the regeneration unit 11 and also reduces theacid gas content of the gas feed before it is passed to thecounter-current column 1 where it is treated with lean amine. The newfeed to the conventional column 1 may now have an acid gas content of,for example, just 1% compared with 2.5% in a typical sour gas feed. Thismeans that the lean amine is used to reduce the acid gas content from alower staring concentration and is therefore focussed on the moredifficult removal. The increased loading of the amine before it ispassed to the regeneration unit 11 means that more of the capacity ofthe amine is used and that therefore flowrates in the regeneration unitare reduced and the supply of fresh solvent to the system as a whole aresignificantly reduced by, for example, 30-50%.

The amine feed from the LPG treatment unit has some hydrocarbonsentrained in it. If passed on to the downstream units, once the sour gashas been separated from the amine, these hydrocarbons can significantlyreduce the efficiency of the units e.g. by de-activating catalysts,blocking the reactors. In addition, they increase the load on the systemin the downstream plants, e.g. the de-sulphurising Claus plant. Thecontactor unit 50 therefore also has the function of flashing off mostor all of the entrained hydrocarbons from the liquid amine stream intothe gas stream. This gas stream may subsequently be passed to aconventional counter-current column where the hydrocarbons will passwith the treated gas to the gas outlet 7. Therefore, when the used aminestream is passed to the regeneration unit 11, there is little or noentrained hydrocarbon to pass to the downstream units. The contactorunit preferably decreases the hydrocarbon content in the amine stream byat least 80%, more preferably at least 90% and most preferably at least95%.

As the LGP treatment unit 20 operates at a higher pressure than the acidgas absorption column 1 and contactor unit 50, there will be no need foradditional pumping and the amine flow from the unit 20 to the additionalcontactor unit 50 can be controlled by the use of remotely operatedvalves (not shown).

As indicated above the contactor unit 50 for de-bottlenecking anexisting process or for selectively removing one or more acid gascomponents from a gas stream and simultaneously flashing offhydrocarbons consists of two contactors or mixers, the second of whichis that shown in FIGS. 3 a to 3 c. The first contactor or mixer may beany of those shown in FIGS. 3, 4 and 5 (including a second mixer of thetype shown in FIG. 3) although any other suitable turbulent contactorsmay also be used. If increased gas flow capacity is required, one ormore contactors may be used either in series or in parallel to theexisting column. The small size of the contactors means that they can beinstalled on existing sites where there would not be room for additionalconventional columns.

FIG. 3 a shows an inline mixer in which a housing 102 for the mixer ispositioned in the pipe 101A, 101B by means of flange connections 103A,103B. The direction of fluid flow through the pipe is indicated by thearrows F1 and F2. This mixer can be easily installed in an existingpipeline without the need for the substantial extra room which would berequired for additional conventional columns. The housing 102 has aninterior wall 121 which is shown as being substantially cylindricalwhich is broken by an inlet opening 122 and an outlet opening 123. Thehousing includes two regulating elements 104 and 105 which are co-axialand have a similar (cylindrical) shape as the housing. These elements104 and 105 are individually rotatable in the housing 102 and eachinclude channels 106A 106B and 107A, 107B in their respective walls. Thechannels preferably have substantially larger length than lateraldimensions. The common axis AX of housing 102 and the regulatingelements 104 and 105 is shown to be oriented at 90° to the general flowdirection although this is not essential. In general, the common axiswill lie broadly transversely to the direction of flow.

At the upstream side, the input flow channels 106A and 106B have aconvergent orientation so that they have a direction generally towards acentral region within housing 102. This is an idealised case. On thedownstream side, the outgoing flow channels 107A and 107B are largelyparallel in a corresponding direction to the through-flow direction. Byrotating the regulating elements 104 and 105 from the position shown inthe figures, the flow channels through the mixer will be changed. Asshown, the channels both upstream and downstream are both aligned witheach other and centred with respect to the openings 122 and 123 so thatfluid flows through with least resistance. This is the mixer in thefully open position where the channels constitute a continuous andedge-free flow path. Rotating one or both of the regulating elements canvary the size and number of channels and thereby affect the fluidvelocity and hence mixing. It will also result in a larger pressure dropdue to the higher flow resistance.

The flow channels (e.g. channels 107A) may be of a circularcross-section as shown in FIG. 3 c or may have an alternativeconfiguration, such as a narrow or slit-like arrangement, for example.The channels can also be arranged so that they have a conical ratherthan a cylindrical form which may introduce a nozzle type effect towardsthe centre of the housing 102. The channels are shown in FIG. 3 c tohave a regular arrangement across the whole of the opening area 122 and123. However, in some circumstances it may be preferable to deviate fromthe regular distribution, in particular at the upstream side of themixer. To increase the capacity of the channels i.e. to reduce theresistance to flow through the mixer, the housing 102 can be designedwith an expanded flow cross section towards one or both opening 102 and103 with corresponding increases in the wall area of the perforatedportions.

Referring to FIGS. 3 b and 3 c it can be seen that the regulatingelements 104 and 105 have coaxial spindles 114 and 115 respectively thecontrol the mutual movement of the elements with respect to each otherand the housing. Adjustment of the relative positions of the regulatingelements will control the flow through the mixer. At one extreme thepassage through the flow channels will be completely closed.

In addition to the channels mentioned above, the regulating elements 104and 105 have bores 104A, 104B and 105A and 105B respectively of adiameter corresponding to the pipe diameter and the openings 122 and123. These bores have an axis lying generally at 90° to the central axisof the respective wall portions with the flow channels. These bores104A, 104B, 105A and 105B can be aligned with the openings 122 and 123to give a substantially free and straight pipe section. Plug-like coremember 112 can be adapted to sealingly co-operate with the internal sideof regulating element 104 at the outer wall 11 2A of the core member.Through the core member 112 there is a bore 112B which preferably liesaligned with and is provided with the same flow cross-section as theinlet opening 122 and the outlet opening 123.

The convergent arrangement of the channels towards the central point ofthe housing 102 gives good mixing for a range of flow patterns. Anyliquid components which are located at the bottom of the flow in pipesection 101A will be lifted by the inclined channels into the centre ofhousing 102. Similarly, any gases which are located in the upper sectionof the pipe in the inlet section are urged downwardly towards thecentral region. The phases therefore mix effectively in the centre ofthe housing 102 and are then fed out in a uniform way through theparallel outgoing flow channels 107A and 107B. There is therefore afully homogenised mixture of the phases across the cross section of thepipe section 101B.

FIG. 4 a shows a first embodiment of a contactor which may be the firstof the series mixers in contactor unit 50. The contactor 201 comprises agas stream inlet 202, a liquid stream inlet 203 and an outlet 204. Thegas stream is supplied to the gas stream inlet which leads to aconverging pipe section 205. The converging pipe section 205 acceleratesthe gas stream as it passes the liquid stream inlet 203 to the end ofthe pipe section 205 where there is a sharp edge 206. Downstream of thissharp edge 206 there is a reaction zone 207 where the gas and liquid arepreferably formed into a homogeneous mixture.

The liquid is supplied to the liquid stream inlet 203 from where it isfed in a controlled manner to the inside of the converging pipe section205. The liquid is presented to the pipe in the form of an annulusaround the inner surface of the pipe. The initial phase velocity of theliquid exposed to the gas stream is governed by the liquid flowrate, thegap distance 208 and the annulus distance 209. The gap distance 208 maybe varied by movement of the blocks 210. The gap will be varied to takeinto account the liquid solvent being used, the properties of which varyconsiderably. The liquid anñulus diameter 209 may be varied by changingthe angle of the converging pipe or by moving the position of the liquidannulus relative to the end of the converging pipe.

The liquid annulus presented to the inner surface of the pipe is drawnalong the inner surface of the pipe in the form of a film 211 by the gasstream. This is best seen in FIG. 4 b. The liquid film 211 closelyadheres to the side of the pipe section 205 until the sharp edge 206 isreached. At this point, the liquid form breaks up to form filaments 212.The generation of the filaments, and their subsequent velocity vector,is determined by the relative velocity between the gas and the liquidphases, the gas-liquid surface tension and the sharp edge 206. Due tothe extremely turbulent conditions in the reaction zone 207, thefilaments 212 are further broken up into very small droplets 213 whichprovide a very high surface area to volume ration thereby makingextremely efficient use of the liquid provided. If appropriate, thisallows the use of considerably smaller volumes of liquid than arerequired by the conventional prior art processes while still achievingsimilar acid gas absorption. The formation of droplets in the reactionzone is favoured by a high Weber (We) number and consequently by a highgas flowrate.

The small liquid droplets and the gas stream are intimately mixed in thereaction zone 207 and the multiphase stream passes on through a conicaldifibser 215 (see FIG. 4 a) where some of the pressure dropped inaccelerating the gas stream in the converging pipe section 205 isrecovered. The multiphase stream then passes on to the second contactor(as shown in FIGS. 3 a, 3 b and 3 c) without separation into separatephases.

FIG. 4 c shows a second embodiment of a contactor suitable for use inthe method of the present invention as the first mixer in contactor unit50. Contactor 220 comprises a gas stream inlet 222, a liquid streaminlet 223 and an outlet 224. The gas stream is supplied to the gasstream inlet which leads to a converging pipe section 225 foraccelerating the gas stream. At the end of the converging pipe sectionthere is a sharp edge 226 downstream of which there is a reaction zone227 where the gas and liquid are preferably formed into a homogeneousmixture. One difference between the contactor of FIG. 4 a and that ofFIG. 4 c is the relative location of the liquid inlet to the annulus ofliquid. In this case, the liquid is supplied to the inlet 223 from whereit passes through the passages 223 a and 223 b to a reservoir 223 cwhich passes round the circumference of the pipe. The liquid then passesout through the channel 223 d which again passes round the wholecircumference of the pipe (see FIG. 4 d) to an annulus on the innersurface of the converging pipe section Because of the shear stressconditions and dynamic pressure exerted by the gas to the liquid, theliquid stream still adheres to the surface of the pipe until the sharpedge 226 is reached.

Another difference between the two contactors is in the slope of theconverging pipe sections 205, 225. In contactor 220 the converging pipesection 225 has a considerably steeper slope than that of contactor 201and therefore reaches a smaller cross sectional area in the same lengthof pipe. The diameter ratio between the throat and the pipe as well asthe angle of the converging cone can be varied independently. Thisreduced cross sectional area will result in a greater acceleration ofthe gas stream as it approaches the sharp edge but will also result in aconsequently larger pressure drop. Also the selection of the angle ofthe converging pipe will be affected by the permanent pressure dropwhich can be accommodated over this apparatus. As indicated previously,the break up of the liquid into filaments and subsequently into dropletsis controlled by the Weber number. This is dominated by the square ofthe relative velocity between the gas and the liquid phases. Therefore asmall change in the velocity of the gas stream, controlled in part bythe acceleration generated by the angle of the converging pipe section,may have a significant effect on the break up of the liquid and hencethe efficiency of the system.

FIG. 4 d shows an enlarged cross section of the area within the circle Aof FIG. 4 c. This shows in greater detail the passage of the liquidthrough the liquid stream inlet 223. The liquid passes through passages223 a and 223 b to a chamber 223 c which passes round the circumferenceof the pipe. The liquid is then fed via the narrow passage 223 d to theinner surface of the conical pipe section 225. The passage 223 d isshown to be very narrow and may be of the order of just 0.2 mm wide. Thepressure drop across this passage is carefully controlled and adjustedto ensure a homogeneous distribution continuous flow of liquid aroundthe whole pipe circumference at the converging pipe section 225. Asindicated above, the size of the passage 223 d is controlled by movementof the blocks 230 and 231. The dotted line 225 a indicates analternative slope for the converging pipe section 225, which gives ahigher gas phase velocity and hence enhanced mixing, but will increasethe permanent pressure drop across the apparatus. This change may beeffected simply by the replacement of one part of the apparatus byanother.

After the gas and liquid have been intimately mixed in the reaction zone227 just downstream of the sharp edge 226, there may be a divergingsection 228 to recover some of the pressure drop. The length of section228 may be varied to control the degree of pressure recovery. Followingthe diverging section 228 there is optionally a considerable length ofstraight pipe to maintain the flow pattern generated and to allowfurther reaction to take place (see FIG. 4 c). The length of thestraight pipe is recommended to be of the order of 15 to 20 standardpipe diameters.

Typical dimensions of the contactors may be in the range of 51-1016 mm(2-40 inches) in diameter. In particular, the apparatus for scavengingof natural gas may have a pipe diameter 216 (see FIG. 4 a) of 610 mm (24inches) with a sharp edge diameter 217 of 253 mm (10 inches). Theinitial diameter 218 of the diverging pipe may be 370 mm (14.5 inches).As stated above, the sharp edge diameter may be varied along with theslope of the converging pipe and other sharp edge diameters which may beused include 296 mm (11.7 inches) similar to that shown in FIG. 4 c and272 mm (10.7 inches).

An example of another contactor which may be used as the first of theseries mixers in contactor unit 50 is shown in FIG. 5 a The turbulentcontactor 300 comprises a vessel 301 having a first fluid inlet 302, asecond fluid inlet 303 and an outlet 304 leading to a venturi passage305. There is a tube 306 (which may or may not be perforated) extendingfrom the outlet 304 back into the vessel 301. The tube 306 may beconnected directly to the fluid inlet 303.

In a first arrangement, the gas mixture is supplied to the vessel 301and the liquid is supplied to the tube 306 optionally directly wherebythe gas is drawn into the venturi by the liquid and the two phases aremixed.

In a second arrangement, the liquid is supplied to the vessel 301 andthe gas mixture is supplied to the tube 306 optionally directly wherebythe liquid is drawn into the venturi by the gas and the two phases aremixed.

In a third arrangement, the liquid and the gas mixture are supplied tothe vessel 301, the liquid being supplied to a level above the level ofthe outlet 304, whereby the gas is forced out through the outlet 304 viathe tube 306, thereby drawing the liquid into the venturi so that thetwo phases are mixed.

A fourth variant is shown in FIG. 5 b. This embodiment is similar tothat shown in FIG. 5 a, but the contactor 310 is inverted. It comprisesa vessel 311 with a liquid inlet 312, a gas inlet 313 and an outlet 314leading to a venturi passage 315. There is a tube 316 (which may or maynot be perforated) extending from the outlet 314 back into the vessel311. The tube 316 may be connected directly to the gas inlet 313. Inthis embodiment the liquid is forced up the tube 316 and the gas isdrawn into the venturi passage 315 by the liquid and the two phases aremixed. When the tube 316 is perforated, the gas may be drawn into thetube 316 through the perforations.

A further example of a contactor which may be used as the first mixer inthe method of the present invention is that shown in FIG. 5 c. Theturbulent contactor 320 comprises a vessel 321 having a first fluidinlet 322, a second fluid inlet 323 and an outlet 324 leading to aventuri passage 325. There is a perforated tube 326 extending from theoutlet 324 back into the vessel 321. The perforated tube 326 is arrangedsuch that there is no gap at the outlet 324 of the vessel 321 for thefluids to pass through. The result of this arrangement is that all thefluid exits the vessel 321 via the perforated tube 326. The tube 326 maybe connected directly to the fluid inlet 323.

In a first arrangement, the gas mixture is supplied to the vessel 321and the liquid is supplied to the tube 326 optionally directly wherebythe gas is drawn into the venturi by the liquid and the two phases aremixed.

In a second arrangement, the liquid is supplied to the vessel 321 andthe gas mixture is supplied to the tube 326 optionally directly wherebythe liquid is drawn into the venturi by the gas and the two phases aremixed.

In a third arrangement, the liquid and the gas mixture are supplied tothe vessel 321, the liquid being supplied to a level above the level ofthe outlet 324, whereby the gas is forced out through the outlet 324 viathe tube 326, thereby drawing the liquid into the venturi so that thetwo phases are mixed.

A fourth variant is shown in FIG. 5 d. This embodiment is similar tothat shown in FIG. 5 c, but the contactor 330 is inverted. It comprisesa vessel 331 with a liquid inlet 332, a gas inlet 333 and an outlet 334leading to a venturi passage 335. There is a perforated tube 336extending from the outlet 334 back into the vessel 331. As for theembodiment shown in FIG. 5 c, the perforated tube 336 is arranged suchthat there is no gap at the outlet 334 of the vessel 331 for the gasmixture to pass through. All the fluids must pass through the perforatedtube 336 to the venturi passage 335.

In this embodiment the liquid is forced up the tube 336 and the gas isdrawn into the venturi passage 335 by the liquid and the two phases aremixed. Since the tube 336 is perforated, the gas is drawn into the tube336 through the perforations.

As indicated above, it is difficult to retrofit multiple feeds toexisting columns and in many instances there is insufficient room to addadditional columms. By introducing the co-current contactor unit 50 itis possible to pre-treat the absorber gas feed using semi-lean aminefrom a variety of sources. Therefore the gas feed stream (e.g. naturalgas) is pre-treated in the co-current contactor unit 50 with anintegrated amine stream. This means that the gas entering the existingcounter-current column 1 has a significantly reduced acid gas contentwhich is then treated with lean amine, The extra loading of the amine bythe recycle means that the amount of lean amine required is reduced byup to 50% and hence the liquid circulation around the system is reduced.

As indicated above, the contactors of the present invention have thesignificant advantage of selectively removing H₂S from a gas stream andsimultaneously flashing entrained hydrocarbon liquids from the solvent.This is particularly useful in situations where H₂S is being removedfrom liquefied petroleum gas (LPG) such as in extractors. Usingconventional columns, hydrocarbons are entrained with the liquidsolvent, reduce the efficiency of the amines and the degree ofabsorption of the H₂S decreases accordingly. Also, LPG flashed in theregenerator occupies capacity of downstream processing facilities suchas Claus plants (sulphur conversion units). By using the method of thepresent invention and feeding it with the solvent from the extractor,i.e. introducing additional co-current contactors before existingcolumns, a large percentage of the H₂S can be removed before theconventional column and hydrocarbons entrained are largely removed. Theefficiency of the conventional column therefore increases and the aminecirculation rate in the column decreases thereby reducing the duty onthe system as a whole.

The amine molecules used in these systems reveal both polar andnon-polar sites, therefore the molecules will interfere with both theliquid hydrocarbon phase and the aqueous phase. For such ahydrocarbon-amine mixture the prediction of the LPG flashing performanceis not possible without experimental data obtained with the fluids inconcern.

The liquid C3+ components in the amine from the LPG-train are not inequilibrium with the C3+ components in the feed gas to be treated. Themain components of the gas are C1, C2 and H₂, and the feed gas isinitially low in C3+ components.

As the contactor unit 50 according to the present invention ischaracterised by a very high gas-liquid interfacial area (per unitlength) and corresponding high mass-transfer rates, it is expected thata multiphase system should approach equilibrium far faster than in acomparable flash vessel. One reason for this is the high driving forceexerted to LPG-liquid exposed to the gas phase. Another reason is thevolume of the LPG-liquid being exposed to the gas phase through thecontinuous redistribution of the LPG-liquid dispersed in the amine. Theexposure and redistribution is enhanced by the high liquid entrainmentand deposition rates associated with the gas-droplet flow and gas-liquidmixing in the contactor unit 50. As a result, the amounts of liquidhydrocarbons are significantly reduced towards the end of the contactorunit 50.

In a flash vessel it is thought that the following assumptions apply:

-   (i) light hydrocarbons (C1, C2) and hydrogen are absorbed or    entrained as gas, and will easily be separated from the amine in the    flash vessel due to the shift in equilibrium conditions (e.g.    pressure) and the favourable conditions for gravitational separation    of the gas bubbles in the liquid;-   (ii) liquid C3+ must be exposed to an atmosphere with sufficiently    low C3+ partial pressure in order to flash. This process is slow,    and the mass transfer depends on gas-liquid interfacial area and the    residence time. Thus the C3+ flash rate in the vessel is assumed to    be low.

In summary, the C3+ mass flux with the amine is a function of theinitial C3+ flux with the amine (prior to the exposure to gascontacting); the gas-liquid interfacial area and residence time in thecontactor unit; and the C3+ gas concentration in the gas mixture

The contactor unit of the present invention will contribute tosignificantly reduced C3+ flux into the flash vessel. Also, amine streamintegration will reduce the circulation rate and hence increaseresidence time in the flash vessel.

FIG. 6 shows apparatus used to test the simultaneous performance of thecontactor unit of the present invention for removal of LPG from theamine feed by flashing during the gas-liquid exposure process, andselective H₂S removal from the refinery off-gas using the semi-leanamine from the LPG extractor. LPG is fed via stream 420 into an LPGextractor 421 together with lean amine via stream 422. The tops productis passed on to a further mixer 423 where additional lean amine isadded. The multi-phase fluid passes on to a settler 424, from wheresemi-lean amine may be fed back to the LPG extractor 421. The gaseousproduct passes from the settler to a coalescer 425 from which moresemi-lean amine is extracted. A significant portion of this semi-leanamine acts as the feed to the contactor unit 430 via stream 426. In thecontactor unit 430 the semi-lean amine reacts with the refinery off-gas(stream 427) producing streams of treated gas 428 and loaded amine topass to the regeneration unit 429.

In order to quantify the amount of LPG removed from the amine stream fedto the contactor, the initial LPG content in the amine stream must bedetermined. Liquid samples of approximately 0.5 dm³ are thereforewithdrawn from the amine stream into an expanding piston cylinder atprocess conditions. The LPG is flashed from the amine sample into apre-vacuumed gas bag by means of repeatedly carrying out the sequence:vacuuming, shaking and settling of the sampled liquid until no more gascan be flashed. The volume of the flashed gas collected in the samplebag is measured, and the composition of the gas mixture is determinedusing gas chromatography.

This analytical method was qualified by making samples of known volumesof purely regenerated amine and mixtures of the regenerated amine andLPG. The method has been further qualified by repetition of the tests.As can be seen from the results shown in FIG. 7, the uncertainty in theliquid analysis is of the order of ±10% which is considered to beacceptable. The results of some of the tests conducted are given inTable 1 below.

TABLE 1 Sour gas Sour gas concentrations (vol %) removed (%) HC removedfrom H₂S in H₂S out CO₂ in CO₂ out H₂S CO₂ amine (%) 2.78 0.58 1.55 1.4679 6 81 2.72 0.61 1.61 1.49 78 7 90

As can be seen from the above results, approximately 90% of thehydrocarbons entrained in the amine and 78% of the H₂S in the sour gascan be removed simultaneously in the contactor unit of the presentinvention. In addition, the co-absorption of the CO₂ is only 7%. Thedata for the second set of results in table 1 is plotted in FIGS. 8 and9.

The present invention therefore provides an affective solution to theproblem of entrained hydrocarbons in an amine liquid feed and alsoprovides an effective method of selectively absorbing H₂S from a gasstream in preference to CO₂ while flashing off most, if not all,entrained hydrocarbons in a recycle integrated amine stream.

1. A method for simultaneously absorbing selected acid gas componentsfrom a gas stream and flashing off hydrocarbons entrained in a liquidsteam including a solvent or reagent for the selected gas component, inwhich: the gas stream and liquid steam are fed into a first contactorwhere they are contacted co-currently and subjected to turbulent mixingconditions; the multi-phase flow from the first contactor is passed to asecond contactor comprising a housing adapted to be inserted into a pipeand to have a fluid flow pass therethrough, said housing comprising aninlet and an outlet opening respectively, the housing being providedwith at least one interior moveable sealingly mounted regulating elementpartially enclosing a central chamber, to provide first wall portionsassociated with an upstream side of said housing, and second wallportions associated with a downstream side of said housing, said wallportions being provided with a number of through-going flow channels,each having a substantially smaller cross-sectional area than the flowcross-section of the inlet and outlet openings respectively, and inwhich the regulating element is adapted to be moveable in relation tosaid housing; and the multi-phase flow from the second contactor isseparated into a gas phase and a liquid phase after the secondcontactor.
 2. A method as claimed in claim 1, in which the firstcontactor is a turbulent contacter having a contacting pipe sectionthrough which a gas steam flows, a liquid inlet arranged to produce anannulus of liquid around the internal perimeter of the pipe, a sharpedge at the end of the contracting pipe and a further pipe sectiondownstream of the sharp edge.
 3. A method as claimed in claim 1 in whichthe first contactor comprises a vessel including a gas inlet, a liquidinlet and an outlet, the outlet leading to a venturi passage and a tubeextending from the outlet back upstream, the tube being perforatedand/or being spaced from the periphery of the outlet.
 4. A method asclaimed in claim 1, in which the first contactor is the same as thesecond contactor.
 5. A method as claimed in any preceding claim in whichH₂S is selectively absorbed in preference to CO₂.
 6. A method as claimedin claim 5, in which the H₂S level in the output gas stream is less than1.5% by volume, preferably less than 1% by volume.
 7. A method asclaimed in any preceding claim, in which the liquid stream is an aminestream including entrained hydrocarbons.
 8. A method as claimed in claim7, in which the liquid stream is fed directly to the first contactorfrom a liquefied petroleum gas (LPG) de-sulphurisation train.
 9. Amethod as claimed in claim 7 or claim 8, in which the amine is selectedfrom MEA, DEIA, DIPA and MDEA.
 10. A method as claimed in any precedingclaim, in which the liquid phase is subsequently treated to remove anyabsorbed gas component.
 11. A method as claimed in any preceding claimin which 70%, preferably 80% and more preferably 90% of the hydrocarbonsin the liquid feed are flashed off into the gas stream.
 12. Apparatusfor the simultaneous absorption of selected acid gas components from agas stream and flashing off hydrocarbons entrained in a liquid streamincluding a solvent or reagent for the selected gas componentcomprising: a first co-current contactor where the gas stream and theliquid stream are subjected to turbulent mixing conditions; a secondco-current contactor comprising a housing adapted to be inserted into apipe and to have a fluid flow pass therethrough, said housing comprisingan inlet and an outlet opening respectively, the housing being providedwith at least one interior moveable sealingly mounted regulating elementpartially enclosing a central chamber, to provide first wall portionsassociated with an upstream side of said housing, and second wallportions associated with a downstream side of said housing, said wallportions being provided with a number of through-going flow channels,each having a substantially smaller cross-sectional area than the flowcross-section of the inlet and outlet openings respectively, and inwhich the regulating element is adapted to be moveable in relation tosaid housing; and means for separating the multi-phase flow from thesecond contactor into a gas phase and a liquid phase.
 13. Apparatus asclaimed in claim 12, in which the first contactor is a turbulentcontactor having a contracting pipe section through which a gas streamflows, a liquid inlet arranged to produce an annulus of liquid aroundthe internal perimeter of the pipe, a sharp edge at the end of thecontracting pipe and a further pipe section downstream of the sharpedge.
 14. Apparatus as claimed in claim 12, in which the first contactorcomprises a vessel including a gas inlet, a liquid inlet and an outlet,the outlet leading to a venturi passage and a tube extending from theoutlet back upstream, the tube being perforated and/or being spaced fromthe periphery of the outlet.
 15. Apparatus as claimed in claim 12, inwhich the first contactor is the same as the second contactor.